1. FIELD OF THE INVENTION
The present invention relates to an air-fuel control apparatus for a fuel injection type internal combustion engine for an automobile.
2. DISCUSSION OF BACKGROUND
It is necessary to maintain an air-fuel ratio in exhaust gas (hereinbelow, referred to simply as an air-fuel ratio) around a theoretical air-fuel ratio in an internal combustion engine, particularly, one provided with a three-way catalytic exhaust gas purifying means. For this purpose, there has been practically used, for instance, an air-fuel ratio control apparatus which comprises an oxygen concentration sensor for detecting an air-fuel ratio on the basis of the concentration of oxygen contained in exhaust gas, an air-fuel ratio control means having an electronically-controlled fuel injection device for controlling the air-fuel ratio of a gas mixture to be supplied to the burning chamber of the engine by controlling the quantity of fuel to be injected, and an electronic control device for effecting feed-back control of a fuel injection quantity by the electronically controlled fuel injection device so that the value of the air-fuel ratio approaches the value of a theoretical air-fuel ratio in response to a calculated air-fuel ratio obtained by an output of oxygen concentration.
In the conventional air-fuel ratio control apparatus, the feed-back control was carried out so that the air-fuel ratio approaches the theoretical air-fuel ratio, whereby an exhaust gas purifying effect could be sufficiently improved by using the three catalytic system disposed in an exhaust gas discharging unit. However, although the conventional air-fuel ratio control apparatus could improve an exhaust gas purifying function, it is difficult to maintain a theoretical air-fuel ratio even under such a condition that the engine is operated in a practically allowable state and the air-fuel ratio is at a lean side, and accordingly the performance of the engine can not sufficiently be obtained. Further, the feed-back control could not be obtained when a high torque is required by rendering the air-fuel ratio to be rich in a full open state of the engine. Accordingly, a precise correction of the air fuel ratio could not be obtained when an air-fuel ratio varies in a rich region because of variation with time and the scattering in dimensional value of the parts of the engine.
In particular, the above-mentioned problem becomes serious in an internal combustion engine with a supercharger. When a predetermined air-fuel ratio in the rich region is further deviated toward a rich side, it falls in an inflammable range to thereby cause firing. On the contrary, when the predetermined air-fuel ratio is shifted toward the lean side, the temperature of the exhaust gas becomes high to thereby cause damage to the parts of the internal combustion engine.
In order to eliminate the above-mentioned problem, there has been such a proposal that an air-fuel ratio for an internal combustion engine is feed-back-controlled to give a predetermined air-fuel ratio, in addition to the control of the theoretical air-fuel ratio, by using a sensor which continuously measures an air-fuel ratio in a region covering the lean side and the rich side on the basis of specified components contained in exhaust gas (such sensor is referred to as an air-fuel ratio sensor hereinbelow).
In the air-fuel ratio control for the conventional apparatus, the information of air-fuel ratio was obtained by calculation of only an output value from the air fuel sensor, whereby the fuel supplying device is controlled to obtain a target air-fuel ratio on the basis of the calculated value of air-fuel ratio information. Accordingly, there was caused an error in the output of the air-fuel ratio sensor due to the conditions (such as the pressure, the temperature and so on) of exhaust gas around the air-fuel ratio sensor when the engine was operated, and such error reduced accuracy in the calculated air-fuel ratio information. Thus, a highly accurate air-fuel control could not be obtained over the entire operable region of the engine.